Paint Edges and Faces separately?
a) First, create a grid mesh object then add a WireFrame modifier.

b) Then adjust thickness as you like at step (1), uncheck the "Replace Original" option at step (2), and change the "Material Offset" to 1 at step (3), this will automatical assign the second material to the edges in the material slot.


c) Now open the material panel, add two the materials to the grid mesh object. The second material automatical will assign to edges or wires due Material Offset option which we set 1 previously in WireFrame modifier.